The invention described herein was made in the course of, or under, United States Energy Research and Development Administration Contract No. W-7405-ENG-48 with University of California.
This invention relates to a method of inhibiting sickling of human sickle erythrocytes, particularly for the treatment of sickle cell disease.
Persons who carry the sickle cell trait have an inherited abnormality of the hemoglobin caused by the mutation of the gene that determines the structure of one pair of the polypeptide chains of the hemoglobin. Sickle hemoglobin (hemoglobin S or HbS) differs from normal hemoglobin in that a single amino acid in one pair of the polypeptide chains has been replaced -- glutamic acid by valine. Sickle cell anemia names the condition for persons having two genes for the abnormal hemoglobin. In this condition, most of the red cells of a sample of fresh blood look discoidal (the normal shape) until deprived of oxygen when the characteristic sickle-shaped forms with threadlike extremities appear. Re-exposure to oxygen causes immediate reversion. Sickle hemoglobin deprives the red blood cells of the ability to transport oxygen; persons afflicted with sickle cell anemia suffer constant sickling with resulting hemolytic anemia. Complications are multiple and severe, e.g. retarded growth, perodic attacks of pain from blood stasis, bone changes, neurological problems, and progressive kidney dysfunction, and increase with age. The complications are due to blockage of the capillary beds in various organs by masses of sickled red cells. Death from anemia, from infections, or, ultimately, from heart or kidney failure often occurs before the age of 35 - 40 years. In persons having only one gene for the sickle hemoglobin, the proportion of normal erythrocytes (red blood cells) is high enough to avoid anemia.
Up to now, treatment of sickle cell anemia has been limited mostly to relieving the symptoms. Recently, a number of compounds acting both covalently and noncovalently has been investigated to modify either the sickle erythrocyte or HbS in an effort to prevent sickling and aggregation of HbS. These modifications include treatment with aspirin, cyanate, anionic pyridoxal derivatives, procaine hydrochloride, organic solvents, protein denaturants such as urea and guanidine hydrochloride, and zinc. To represent a potentially useful antisickling agent, a compound must be effective in concentrations compatible with erythrocyte physiology. Furthermore, agents acting on HbS must be able to readily penetrate the erythrocyte membrane. Several of the aforementioned compounds must be employed at high concentrations. Moreover, although direct pyridoxylation increases the solubility of HbS, the alkylating agents employed have been shown to penetrate membranes poorly, resticting the interaction of these compounds with hemoglobin when added to cells.
More recently, dimethyl adipimidate (hereinafter referred to as DMA), a bifunctional crosslinking reagent that is known to link covalently the free amino groups in polypeptides, was reported to inhibit sickling in vitro (B, H, Lubin et al, "Dimethyl Adipimidate: A New Antisickling Agent," Proc. Nat. Acad. Sci. USA 72(1), pp. 43-46, January, 1975). Although bifunctional imidates such as DMA have been found to prevent sickling at very low concentrations, these reagents have also been found to be deleterious to several membrane systems; hence, the potential exists for adverse side effects when used as therapeutic agents for the treatment of sickle cell disease.